CA1073100A - Dynamic focussing of video disc objective lens - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to an improvement in a video signal playback device which derives video signals from a track on a video disc using a light source impinging upon an information track on the surface of the video disc, the light beam modulated by the information track being sensed in a light sensor. In prior art devices, poor picture quality has resulted from the fact that focus of the beam on the information track cannot be maintained dynamically, that is as the turntable rotates and the disc and read head move relative to one another radially of the disc. The present invention overcomes the deficiencies of the prior art in this respect by providing a reader and disc arranged for relative lateral movement radially of the disc wherein an objective lens mounted in the disc reader adjacent the disc maintains focusing of the light beam on the information track of the disc as the disc is played. The objective lens is mounted for movement relative to the surface of the disc along the path of impinging light beam to maintain focus of the beam on the information track as the rotating disc moves laterally relative to the reader.

Description

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' DYNAMIC FOCUSING OF VIDEO DISC OBJECTIVE LENS
BACKGROUND OF THE PRIOR ART
Systems have heretofore been developed for reproducing signals at video frequencies from information recorded on discs, tapes, or other media~ Such systems have utilized, among other things, optical recordings upon photosensi-tive discs, electron beam recording on thermo plastic surfaces and, in prior patents assigned to the assignee of the present invention, systems utilizing a rotating disc which is responsive to impinging radiation to reflect or transmit radiation corresponding to and representative of the information stored on the surface of the disc.
For example, in U. S. Patent No. 3,530,258, issued - to David Paul Gregg and Keith O. Johnson on September 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair of flexible, fibre optic elements. An air bearing supported an objective lens system. A light source of radiant energy was positioned below the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source which directed an energy beam to the surface of the disc and provided a transducer that was responsive to reflected energy.
One of the major problems to be encountered in the recording and reproduction of video information, arises directly from a con-sideration of the energy levels involved in such a process and lC ~

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To be commercially desirable as a home instrument, the system should be able to store and reproduce a 'Iprogram'' of at least 15 to 30 minutes in length. The record disc should be of an easily handled size, comparable to the phono-graph records currently in use. If the playback turntable was operated at 1800 rpm, some 54,000 revolutions would provide 30 minutes of playback. Assuming a 1 micron track width and 1 micron spacing between adjacent tracks, a circular band approximately 4.25 inches wide is required. Assuming that the smallest radius at which information can be stored is approximately three inches, the resultant disc is about 15 inches in diameter. The duration of the program or the speed of the turntable can change the dimensions of the recorded area, as can the width of the individual track and the spacing between adjacent tracks.
Assuming that the video information has been recorded in some digital fashion, the presence or absence of a signal can be detected at an appropriate information rate. If the width of the track is approximately one micron, and that the space between adjacent tracks is also one micron, the quantity of energy necessary to impart information from the disc can be determined. It is necessary to provide sufficient radiant energy to "illuminate" a "spot" of approximately one micron in diameter and, at the same time, provide sufficient radiant energy at the detector, so that the 'Ipresence" or "absence" of a signal can be distinguished.
It has been discovered, in attempting to utilize the ransmitted radiation techniques of the prior art, that the ~ -2-cbr/

- ~7310~1 provision of an inordinately large amount of radiation into the system is required in order to "transmit" a sufficiently useful increment of energy for detection through the record.
It has also been determined that a substant:ial magnification S is -2a-cbr~3~.

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re~u.ired to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a light source illuminates the entire field which can be scanned by the detector under control of the servo system, it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity -to register upon the photosensitive device.
According to the present invention, there is provided an apparatus for use in a video disc playback systemJ
the system including a video disc reader and a turntable adapted to receive and rotate a video disc, the reader having a light beam source for impinging upon an information track on the surface of the video disc and a light sensor for xeceiving light refl.ected from the disc, -the reader and disc arranged for relative lateral movement radially of the disc.
The apparatus has an objective lens mounted in the disc reader adjacent the disc for focusing the light beam on the information track of the disc and passing the reflected light to the sensor. Means is provided for moving the objective lens relative to the surface of the disc along the path of the impinging light beam to maintain focus of the beam on the information track as the rotating disc moves laterally relative to the reader.
In a specific embodiment of the invention, the read head is yieldably mounted on a head support and is adapted to be maintained in close proximity to the video disc surface, the objective lens being moun-ted on the read A sb/ ~

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head for movement therewith. The means for moving the objective lens may include adjustable bias means for applyiny a varying force to the read head in a direction normal to the disc to achieve and maintain a predetermined spacing between the head and the disc substantially independent of the radial position of the head.
According to another aspect of the present invention, there is provided a method for readiny information recorded on the surface of a video disc, the method including steps ::
of rotating the video disc, impinging a light beam upon an information track on the surface of the disc, and sensing reflected liyht received from the disc while the reader and disc move relative to one another radially of the disc. The liyht beam is focused on the information track of the disc through an objective lens adjacent the disc, and the reflected light passes throuyh the objective lens to the sensor. The objective lens is moved relative to the surface of the disc : .
alony the path of the impinginy light beam to maintain focus of the beam on the information track as the rotating disc moves relative to the reader.
More specifically, the method of the present invention .includes the step of providiny a read head yield-ably mounted on a head support and adapted to be maintained in close proximity to the video disc surface, the objective lens being adapted for movement with movement of the read head. A varying force may be applied to the read head in a direction normal to the disc to achieve and maintain a pre-determined spacing between the head and the disc subs-tantially sb/~

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independent of radial position of the head.
An important aspect oE the present inven-tion is the abili-ty to direct the illuminating radia;tion to a particular spot and to return the information from the spot thus illuminated to a detector system. T;he prior art has suggested the use of a pair of transducers in conjunction with a summing amplifier to provide signal information and a differential amplifier to provide feedback servo information for error correction. However, given the limitations of the extremely low radiation levels, the diffrac-tlon limited characteristics of the image and the extreme sensitivity of the system to noise and vibration, such an approach is not entirely satisfac-tory. A difference "curve following"
technique described in the patent to W.D. Munro, ~.S.
Patent No. 2,838,683, issued June 10, 1958, has suggested an alternative solution. .
Accordingly, it is an object of the present invention to provide an improved playback assembly for a disc upon which video information has been recorded.
It is yet another object of the present invention to provide an impxoved tracking circuit for optically scanning a video disc.
It is yet another object of the invention to provide an improved scanning assembly for video disc which includes an optical system for directing a radiant energy spot to the disc and to detect reflected radiant energy therefrom and to direc-t this reflected energy to a photo-sensitive transducer.

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It is yet an additional object of the invention to provide an improved articula-ted mirror assembly in the optical path between a light source and the surface of the video disc, which mirror assembly can be used to direct the location of the spot relative to the disc surface within certain limits.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection wlth the accompanying drawings in which several pre~erred embodiments of the invention are illustrated by way o:E example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF TIIE DRAWINGS
FIG. 1 is an idealized side view of a playback assembly according to the present invention;
FIG. 2 is a more detailed block diagram of the elements in the optical playback system;
FIG. 3 is an idealized v.iew of an alternative articulated mirror assembly;
FIG. ~ is a block diagram of a suitable detector and tracking circuit;
FIG. 5 is a block diagram of an op-tical detector of the prior art suitable for use in the present invention;
FIG. 6 is an enlarged side view of -the opti.cal head and air bearing assembly;

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FIG. 7 is a top idealized view of a cam and follower assembly for controlling the bias on the air bearing assembly; and FIG. 8 is a side view vf another alternative articulated mirror arrangement useful in the system of the presen-t invention.

DETAILED DESCRIPTION OF THE INVE'NTIO_ Turning first to FIG. 1, there is shown, in side view, a playback assembly 10 suitable for use in the present invention. The playback assembly 10 includes a laser element 12 which moves wi-th the playback assembly 10. It is, however, within the state-of-the-art to provide a stationary laser which is coupled optically to the movable assembly 10. Preferably, the laser 12 provides coherent, polarized light. A read head 14 is mounted in arm 16 of the playback assembly 10.
A video disc 2Q, which has video information recorded upon it is mounted on a turntable 22, which ls adapted to rotate the disc 20 at a relatively high speed.
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speed is set a-t 1800 rpm.
Suitable video discs have been described and claimed in the patents to Gregg, Johnson, supra.
The playback assembly 10 is mounted on a rotatable element 24 which, in the view of FIG. 1, translates the readîng head in the radial direction relative to the disc 20 and in an arc that is generally orthogonal to the plane of the drawing.
The laser 12 generates a reading beam 26 which generally passes from the laser 12 -through an optical sys~em to the playback head 14. The beam is then directed to the surface of the disc 20 and returns through the playback head 14 along the same optical path until a read assembly 28 is encountered. The read assembly 28 is mounted on the arm 16.
In operation, the laser directs a reading light heam 26 to the surface of the disc 20 through the optical sy~tem.
The information recorded upon the disc interacts with the impinging beam and a reflectçd beam is produced which contains the recorded information. The reflected light beam is returned to the optical system which "analyzes" the returned beam to determine whether the beam is properly tracking the signal channel.
If the electronics determine that the laser spot is not being directed to a predetermined area of the information channel, appropriate servo signals are derived which, when applied to the read head 14, cause the point of impingement of the laser beam to shift in the radial direction to retain alignment with the track that is being read.
In an alternative embodiment, the driver for the rotatable element 24 for the playback assembly 10 can also be 3Q controlled by the servo signals which changes the position of cbr/~5~

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the laser spot. In yet other embodiments, a motor can be coupled to the turntable driver to provide a predetermined increment of radial motion for each revolution of the turntable 22. In any case, the playback head D can be made to track the information channel recorded on the disc ?0 with a "coarse" adjustment being applied to the d:river of the rotatable element 24 and a "fine" adjustment being applied to an articulated mirror, described in ~reater detail below.
Turning next to FIG. 2, there is shown a diagram of the ~lements of the reading system. The reading laser beam 26 is applied to a beam splitting prism 30. The prism 30 is rotated slightly with respect to the optical path. A lens 32 is provided to better form the beam 26 at the surface 20 and to optimize the resolving power of the system. The transmitted portion of the beam 26 is applied through a quarter wave plate 36 and is then directed through the reading head 14 to the disc 20.

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A returning keam 38 containing the information from the disc 20 follows substantially the identical path. At the quarter wave plate 36, the returning beam is now given an additional quarter wave shift for a total polarization of one-half wavelength. The returning beam 38 reaches the beam splitter 30 and is reflected therefrom to a suitable optical system 40. Light from the laser 12 that is initially reflected in the prism 30 and re-reflected ~rom the base of the prism will, due to the slight rotation of the prism 30, be aimed at a point that wholly misses the detector 40. Moreover, the cumulative effect of the quarter wave plate which polarizes the returning beam by ~ 2 substantially attenuates any trans-mitted component. What is transmi.tted is cross polarized with : _9_ cbr/~

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respect to the laser 12.
The read head 14 .includes a fluid-bearing member 50 which is adjacent to and supportive of a microscope objective lens 52. A limited amount of vertical adjustment is available in the objective l~ns 52. Directing the illumination to the objective lens 52 is an articulated mirror 54 which is mounted adjacent to and cooperates with a second or fixed mirror 56 that is substantially paxallel with the articulated mirror 54.
The fixed mirror receives the reading beam 26 and directs it to the articulated mirror 54.
The reading beam 26 undergoes at least one reflection from the articulated mirror 54 before the beam is applied to the objective lens 52. Two such reflections are illustrated in the embodiment of FIG. 2. Similarly, the beam path is such that a reflected beam 38 returning from the surface of the disc 20 would also undergo two reflections from the articulated.
mirror 54 and two reflections from the fixed mirror 56 before proceeding into the optical path including an additional fixed : mirror 57 which ultimately leads to the read assembly 28. .
In the embodiment illustrated, the articulated mirror 54 is mounted on a point pivot 58 that is centrally located with respect to the mirror 54. The mirror 54 may have an oblong shape with the long axis in the plane of the drawing and the short axis orthogonal to the plane of th~ drawing. As shown, a mirror driver 60 is connected to one end of the machine 54 and is operable to impart motion about the central pivot 58.
.If the driver 60 rotates the mirror 54 in the clock~
wise direction, as viewed in FIG. 2, the point of impingement of the read beam 26 will be shifted to the left. This would cbr/;~ , 73~

represent a deflection of the beam in a first radial direction.
If the driver 58 rotates the mirror 54 in the counter-clockwise direction, then the point of impingement of the transmitted beam 26 will be shifted to the right, as seen in FIG. 2, or in a second, opposite radial direction.
It will be obvious that the reflected beam 38 and the reading beam 26 trace identical paths between the surface of the disc 20 and the beam splitter 30. The articulated mirror 54 serves to "steer" the reading spot to a desired location and then "reads" only the illuminated area, trans~
mitting that information back to the read assembly 28.
In alternative embodiments, the articulated mirror 54 and the stationary mirror 56 can be adjusted and repositioned to provide a greater plurality of reflections between the two mirrors before the beam continues either to or from the di~c surface 20. In such an arrangement, the magnitude of mirror deflection required to steer the reading spot appropriately can be greatly reduced~ The driver 60 therefore, need only impa~t small, incremental motions to the articulated mirror 54.
In an alternative embodiment, as shown in FIG. 3, a first articulated mirror 54' is provided which is mounted on a central pivot member 58', and is driven about an axis ortho-gonal to the plane of the FIGURE and in the clockwise and counter-clockwise direction by a first driver 60' that is coupled to the mirror 54' at the end of a long axis.
A second driver 60" is coupled to one end of a third mirrox 54" for imparting rotational motion to the third mirror 54" about the long axis that is in the plane of the FIGURE, In operation, the first driver 60' permits translation of the beams in the "radial" direction to permit "fine" tracking --11~
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of the inormation channel. The second driver 60" is used to translate the beam in the circumferential direction, to provide time synchronization, if d~sired, and to compensate for eccentricity.
In other embodiments, the problem of time synchron-ization can be handled mathematically, as a ~tep in the process of electronically compensa-ting for eccentricity of the disc 20 and in such embodiments~ only the single articulated mirror is used.
Turning next to FIG. 4, there is sho~n a preferred embodiment of the optical detector assembly 40 which utilizes some of the electronics of the Munro patent, supra. As shown in FIG. 4, the returned optical image 38 is directed to impinge upon a photocell 70 when a channel is being tracked properly, with the spot on the outer half of the track, a predetermined output signal is generated. The output of the photocell 70 is applied to a comparator 72. An adjustable bias 74 is applied to -the other input of the comparator 72 and is adjusted to provide a null when the predetermined output signal is being applied. The error signals resul-ting from drift can be inte-grated, and the outpu-t of the integrator can be applied to an appropriate circuit to urge the movable playback assembly 10 relative to the center of the disc 20. The error signal is also used to apply a signal directly to the mirror driver 60 of FIG. 2 to urge the beam to follow the track.
If, however, the track is not being followed properly, depending, of course, upon the characteristics of the disc surface, a condition will be presented in which the energy impinging upon the photocell 70 will be different than the bias provided by bias circuit 74, and accordingly, the error -~

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signal of appropriate polarity will be provided to correct the position oE the light spot relative to the information channel. The integrator output then is applied to the movable playback assembly 10, and if the bias signal i9 greater, a forcing function is generated tending to send the SPot toward the periphery of the disc. If the received signal is greater, the spot is directed to the center of the disc. As the spot follows the spiral track properly, the differential output tends toward the null.
In FIG. 5, there is illustrated the prior art optical detector eIectronics utilized and shown as FIG. 10 in the previously issued Gregg, et al., U.S. Patent No. 3,530,2S8, assigned to the assignee of the present invention. For convenience, the same reference numbers are used in Gregg, et al and herein. A pair of photo detectors 96, 98 are employed which, in combination, provide an additive information signal and, when diferenced, an error signal which controls servo elements that redirect the reading elements. As applied to the present invention, the radial error signal could be applied to either of the drivers 60, 60' of the articulated mirror assemblies of FIGS. 2 and 3, respec-tively.
As shown in FIG. 5, a dual photo detector has two sections 96, 98 whose outputs are applied to respective amplifiers 100, lOl. The out~uts of the amplifiers lO0, 101 are summed in a summing network 106. The output from the summing network represents the sum signal from the two photo detector sections 96, 98 and constitutes the modulated signal output of the transducer~
The signal amplitude from the first photo detector bm.

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section is applied to a detector 102, and this detector produces a negative unidirec-tional signal representative thereof. The signal amplitude from the second photo detector section is applied to a detector 103, and the latter detector produces a negative unidirectional signal in response thereto.
The two signals are added algebraically in a summing network 105 which produces an error signal.
In the present example, th~ resulting error signal is amplified in an ampli~ier 104, and it is applied to the circuits of FIG. 3 and.driver 60'. The error signal applied to the driver 60' causes the mirror 54' to shift the beams in a radial direction with respect to the disc 20, as explained above. The direction and amount o the shift depends on the polarity and amplitude o the error signal, so as to maintain the spot in perfect registry with the recording track on the record 20.
The output signal from the summing network 106 is applied to appropriate video detection and reproducing circuitry such as is illustrated in FIGS. 17 and 18 o-f Gregg et al, supra, and described therein.
The DC component of the output of the amplifier 104, when properly processed, may be used in several ways to move the pick-up arm of FIG. l,across the disc 20 at very nearly the rate which makes the signal approach zero. One method is to integrate this component over short intervals until it reaches a predetermined value, at which it triggers a solenoid.
This solenoid, in turn, actuates a light-duty friction ratchet -r~
which then turns the pick-up arm through a very small angle as is taught in Gregg et al, supra.

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: Another method also suggested in Gregg et al, supra, is to use an inexpensive elec-tric clock movement with a reduction year to drive the arm continuously across the disc at a rate just slightly above 2 microns for each 1/30 second or revolution of the disc. In this case, the integrated signal of the first method is used to interrupt the motor voltase occasionally. To assist the proce$s, the arm 16 of FIG. 1 may be biased slightly towards the center of the disc 20.
In FIG. 6, there is shown an enlarged side view of the lens and air bearing assembly of the playback head 14.
The movable arm 16 connects to the playback head 14 through a pair of parallel leaf springs 120, 122. The spring force of the leaf springs 120, 122 is generally insufficient to maintain the springs in the horizontal position ~ith the playback head 14 unsupported by the fluid bearing that is generated by the rotating disc 20. Within the read head 14 is the fluid bearing member 50 and the microscope type objective lens 52. Also contained in the read head 14 are the fixed and articula-ted mirrors 54, 56, 57 necessary to direct the beam of light from the source to the lens 52 and back from the surface of the disc 20.
A support post 124 extends outward of the read head 14 toward the inner end of the arm 16. Mounted to this support post 124 is a bias spring 126, the other end of which is fastened to a lever 128. The lever 128 is coupled to the arm 16 and, through a flexible cable 130, connects to a cam and follower assembly 132, to be described in connection with FIG.
7, below.
Also included, but not described in detail, are .

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appropriate interlocking solenoid assemblies opexating in conjunction with the cam and follower assembly to maintain the read head 14 out of contact with the clisc 20 as the arm 16 swings out of engagement with the disc 20, and whi.ch act to prevent damage if, for any reason, the disc 20 should slow appreciably while being tracked by the read head 14.
The bias spring 126, ~hen compressed, acts like a solid rod~ enabling the lever 128 to directly cam the read head 14 upward and away from the disc 20, if this configuration is - 10 desired. Alternatively, when the read head 14 is in position over the disc, the lever 128 rotates in the opposite direction, relieving the compression on the spring 126. ~nder normal cir-cumstances, the weight of the read head 14 is supported by the fluid bearing member 50 on the disc, thereby enabling the leaf springs 120, 122 to be substantially parallel and horizontal.
According to the present invention, an additi~nal bias is provided through t-he use of the bias spring 126 to maintain a substantially constant separation between the read head 14 and the fluid bearing member 50 and the surface of the disc 20. The relative surface velocity changes as the moving arm 16 progresses toward the center of the disc and the fluid bearing is less able to support the read head. Therefore, at the outset, the lever 128 is rotated in the downward direction, applying a stretch to the spring 126 which, in turn, imparts a downward force to the support arm 124, thereby increasing the bias on the fluid bearing 50 while the fluid pressure is at its greatest.
As the arm 16 moves inwardly of the disc 20 and the surface velocity is reduced, a cam follower arrangement bm.

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3~10 gradually rotates the lever 128 in the upward direction, reducing the tension of the spring 126, thereby lessening the bias on the read head 14. By selecting an appropriate cam contour, the bias on the fluid bearing 50 can be maintained at an optimum value for constant separation from the disc 20 for the surface velocity of the disc at any radial location.
Turning now to FIG. 7, there is shown one form of cam and follower assembly 132 that can drlve the lever 128 through the flexible cable 130 (also shown:in FIG. l)q A cam 140 is cut so that at the outermost position of the arm 16, a follower 142 rests on a high lobe which maintains the head 14 in an "up" position, safely out of contact with the edge of the rotating disc 20.
As the arm 16 tracks inwardly., the follower 142 immediately proceeds to the innermost point on the cam 140 surface, applying maximum bias to the read head 14. As the arm then continues inwardly in the radial direction, the follower 142 gradually rides outwardly from the center of the cam 140, thereby reducing the bias forces on the read head 14.
. It is clear that tecnniques are readily available fo~ transmitting simple mechanical motion from the cam follower assembly 132 to the arm 16, and the specific details are unnecessary in the present applicationr In FIG. 8, there is shown an alternative configuration - for the articulated mirror assembly that is mounted on the read head 14. In this alternative embodiment, a fixed mirror 150 and an articulated mirror 152 are arranged on converging planes.
An incoming beam in the horizontal direction impinges upon the articulated mirror 152, and through multiple reflection between bm.

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the fixed mirror 150 and the articulated mirror 152, the beam is ultimately rotated through 90 and is directed down-ward into the reading assembly. Similarly, the returning beam retraces the same path. The mirror 152 is articulated to rotate about an axis that is in the plane of the drawing to deflect the transmitted beam in a direction that is perpendicular to the plane of the drawing.
The angle of incidence of the mirror 150 and the angle of convergence between the mirrors 150 and 152 are controlled so that the incoming beam makes a plurality of reflections off of the two mirrors before being directed into the disc. Moreover, since the pair of mirrors, in addition to providing a "folded" light path, also rotates the beam through 90, a separate 45 mirror can be omitted, thereby increasing the intensity o available light to the disc. Of course, this would permit at least one extra reflection between the mirror pair without in any way degrading the quality of the light beam.
The same number of internal reflections as in the embodiment of FIG~ 2 could be employed wi-th less light loss in the mirror system.
Thus, there has been shown an improved video disc reading assembly which steers the illuminating radiation to the information track on the surface of the disc and steers the return signal from the track to an optical detector. An articulated mirror enables the steering of both the transmitted and the returned light beam.
An improved optical detector is utilized in combination with a fixed bias source so that a single detector provides both the information signal and the servo signals .

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necessary to track the information channel.
A novel air bearing assembly has also been dis-closed, which enables a microscope lens to tra~el at a fixed distance above the disc supported on a fluid bearing, and means are provided to impart a variable bias to the fluid bearing as a function of relative velocity between the disc 7 and the bearing member.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for-use in a video disc playback system, the system including a video disc reader and a turn-table adapted to receive and rotate a video disc, the reader having a light beam source for impinging upon an information track on the surface of the video disc and a light sensor for receiving light reflected from the disc, the reader and disc arranged for relative lateral movement radially of the disc, said apparatus comprising: an objective lens mounted in the disc reader adjacent the disc for focusing the light beam on the information track of the disc and passing the reflected light to said sensor; and means for moving said objective lens relative to the surface of the disc along the path of the impinging light beam to maintain focus of the beam on the information track as the rotating disc moves laterally relative to the reader.

2. Apparatus as claimed in Claim 1, including a read head yieldably mounted on a head support and adapted to be maintained in close proximity to said video disc surface, said objective lens mounted on said read head for movement therewith.

3. Apparatus as claimed in Claim 2, wherein said means for moving said objective lens includes adjustable bias means for applying a varying force to said read head in a direction normal to the disc to achieve and maintain a predetermined spacing between said head and said disc sub-stantially independent of radial position of said head.

4. Apparatus as claimed in Claim 3, including at least one resilient member for mounting said read head to said head support, said resilient member being insufficient to support the weight of said head.

5. Apparatus as claimed in Claim 4, wherein said adjustable bias means includes bias control means coupled between said head support and said read head and operable in response to read head radial location for varying the supporting force applied to said read head to compensate for different supporting forces contributed by said fluid bearing due to different surface velocities at different radii of the disc.

6. Apparatus as claimed in Claim 5, wherein: said resilient member includes at least one leaf spring extending between said head and said head support; said adjustable bias means includes a bias spring coupled to said head; and said bias control means includes a cam and cam follower connected to said head support and coupled to said bias spring for varying the bias imparted to said head by said bias spring as a function of the cam follower position on said cam, the follower position being determined by the read head location relative to the center of the disc.

7. Apparatus as claimed in Claim 1, including a laser beam source mounted in the reader for producing a reading laser beam utilized in playback of the video disc.

8. Apparatus as claimed in Claim 6, wherein said cam and cam follower are arranged to increase the supporting force to said head as said head moves radially inwardly of the disc.

9. Apparatus as claimed in Claim 7, wherein said apparatus includes optical means for directing said reading laser beam along a path from said source means through said objective lens to said video disc.

10. Apparatus as claimed in Claim 9, wherein said optical means includes path adjusting means for selectively altering the path of said reading beam in relation to said objective lens.

11. Apparatus as claimed in Claim 10, wherein said path adjusting means includes an articulated mirror in said path for altering the path of said reading beam in relation to said objective lens to cause said reading beam to shift laterally of said video disc.

12. Apparatus as claimed in Claim 11, wherein said path adjusting means includes a fixed mirror disposed in said path in opposing spaced relation to said articulated mirror to provide more than one reading beam reflection therefrom.

13. Apparatus as claimed in Claim 6, wherein said bias control means includes a cable means for coupling said cam follower to said bias spring.

14. Apparatus as claimed in Claim 6, wherein said bias spring is a compression spring capable of bottoming when compressed, and wherein said bias control means is adapted to selectively compress and bottom said bias spring to enable said cam and cam follower to cam said read head directly.

15. An improvement in a method for reading information recorded on the surface of a video disc, the method including rotating the video disc, impinging a light beam upon an information track on the surface of the disc, and sensing reflected light received from the disc while the reader and disc move relative to one another radially of the disc, said improvement in the method comprising: focusing the light beam on the information track of the disc through an objective lens adjacent the disc and passing the reflected light through the objective lens to the sensor; and moving the objective lens relative to the surface of the disc along the path of the impinging light beam to maintain focus of the beam on the information track as the rotating disc moves relative to the reader.

16. The improvement in the method as claimed in Claim 15, including providing a read head yieldably mounted on a head support and adapted to be maintained in close proximity to the video disc surface, the objective lens adapted for movement with movement of the read head.

17. The improvement in the method as claimed in Claim 16, including applying a varying force to the read head in a direction normal to the disc to achieve and maintain a predetermined spacing between the head and the disc sub-stantially independent of radial position of the head.

18. The improvement in the method as claimed in Claim 17, wherein said step of impinging a light beam upon an information track of a video disc includes adjusting the optical path for selectively altering the path of the light beam in relation to the read head.

19. The improvement in the method as claimed in Claim 18, wherein said step of impinging a light beam upon an information track of a video disc includes adjusting the optical path of the reading beam in relation to the read head to cause the light beam to shift laterally across the video disc.

20. The improvement in the method as claimed in Claim 17, wherein said step of moving the objective lens includes selectively directly camming the read head, and thereby the objective lens, away from the disc.